Substantial interest in thermally stable polyimide, polyamideimide, and polyesterimide resins has been shown in recent years. Polyimides are characterized by high thermal stability, outstanding resistance to irradiation, to mechanical deformation at high temperatures, and to solvent attack, good hydrolytic stability, and an excellent balance of mechanical and electrical properties over a broad temperature range. Aromatic polyimides are superior to aliphatic polyimide compositions, in that they have higher heat stability.
The reaction product of pyromellitic dianhydride with bis(4-aminophenyl)ether is an example of a known aromatic polyimide having good thermal stability. This polyimide is stable to over 500.degree. C in vacuum or inert atmospheres, stable for over one year when stored in air at 275.degree.C, retains toughness after one year in boiling water, and retains flexibility after 40 days exposure to a thermal neutron flux of 10.sup.13 neutrons/cm.sup.2 /sec at 175.degree.C. The only known solvent for this polyimide is fuming nitric acid. The mechanical properties of glass reinforced composites using polyimide resins are generally good. Some such composites retain their flexural strength at 600.degree.F after extended aging at 600.degree.F and 100 hours at 700.degree.F in air. Commercially available polyimide glass cloth reinforced composites have been reported to retain mechanical properties after more than 10,000 hours storage at 400.degree. and 500.degree.F in air. Polyamideimides are structurally similar to polyimides, but contain amide linkages to enhance processing characteristics. Polyesterimides exhibit good physical and electrical properties at high temperatures and a 20,000 hour service life at 230.degree.C. The thermal and oxidative stabilities of such polymers are somewhat less than those of the best polyimides, but are still far superior to other current thermally stable polymers.
Polyimides, polyamideimides, and polyesterimides reported in the literature are prepared by either (1) melt or fusion techniques or (2) cyclization of soluble polyamic acid precursors. Regardless of the approach used by-products are formed, volatile or otherwise. Diamines and tetraacids or diamines and diacid/diesters can be reacted by melt or fusion techniques to produce meltable polymeric polyimides wherein the backbone contains the imide linkages through the following route: ##SPC1##
While here the Diacid/Diester is used, the tetraacid gives the same desired end product.
A more general method for the preparation of aromatic polimides and various alipatic polyimides is now in wide use. This method involves the synthesis of a soluble polyamic acid, which can be converted to the polyimide by heating at elevated temperature.
These polyamic acids are prepared by the reaction of a dianhydride with a diamine, as shown below in Prior Art Reaction IIA. ##SPC2##
First the polyamic acid solution is heated to remove solvent, and then elevated in temperature to about 300.degree.C, at which complete cyclization occurs to form an insoluble polyimide and water. FNT *Dimethylformamide ##SPC3##
R in IIA and IIB is an alipathic group from 1- 20 carbon atoms.
According to the present invention, it has been discovered that imide-oxirane reaction products can be produced by the reaction of primary cyclic mono, di- and polyfunctional imides with oxirane compounds in the presence of active chromium III salts having unoccupied coordination sites. These reaction proceed rapidly at temperatures above about 0.degree.C, while state of the art polyimide preparation techniques require temperatures of at least about 300.degree.C. In the instant invention there is no production of water or any other by-product.
The reactions of the instant invention may utilize either monofunctional or difunctional or higher functionality oxiranes and monofunctional, difunctional or higher functionality cyclic primary imides. The reaction is illustrated here by the use of a primary cyclic diimide with a difunctional oxirane, in the presence of an active chromium III catalyst. ##SPC4##
where R is aliphatic, aromatic, cyclo aliphatic, aralkyl, alkaryl as well as the above moieties with non-reactable substituents such as halogens, cyano, ether, ester, amide, imide, etc., thereupon and y is 1 or 0, and n is an integer of at least 2.
Since no by-products are evolved, these polymers are amenable to standard bag molding techniques for preparing fiber reinforced structures, thus reducing fabrication costs and opening the application of these polymers to large structures. These polymers are also useful as adhesives, where the absence of by-products eliminates the need for high pressure for bonding. They can also be used in simple potting operations where only heat is required to obtain even large castings for encapsulation of electronic components.